This project examines mechanisms of pathogenesis of obligately intracellular bacteria. Initial events of Chlamydia-host interaction, including attachment, internalization, inhibition of phagosome-lysosome fusion, and differentiation are examined. Study of the attachment and penetration phases of chlamydia with eucaryotic cells required an efficient means of distinguishing bound from internalized parasites. We assayed agents that had been used to release viruses or polypeptide hormones from the cell surface and found that heparin most effectively released bound chlamydia. The technique of heparin release was coupled with temperature shift experiments to examine a number of treatments of chlamydia or host on the uptake of chlamydial elementary bodies. We have identified two chalmydial proteins, present on the infectious stage of the life cycle but absent from the noninfectious form, that bind host cell surface protein. These two proteins also bind heparin, are sensitive to reducing agents and/or protease inhibitors and vary between Chlamydia trachomatis serotypes in correlation with disease caused. These proteins possess a number of properties that, collectively, are suggestive of a role in host-parasite interaction. A different approach has been taken in the study of Coxiella burnetii. This rickettsia undergoes a serologically defined virulent to avirulent phase transition. Our studies have demonstrated that the component that is structurally and anti-genically unique between phases is the lipopolysaccharide. Consideration of the serological definition of phase in the context of LPS variation allowed us to identify a previously unknown LPS chemotype intermediate in structural complexity to the virulent and avirulent LPS types. This intermediate type LPS should prove useful in understanding the structure and function of C. burnetii LPS.